1. Field of the Invention
The present invention relates to optical systems. More specifically, the present invention relates to nonreciprocal optical elements.
2. Description of the Related Art
Nonreciprocal optical elements (NOEs) are optical devices with different conditions of beam propagation in opposite directions. They are necessary components for applications in many areas, e.g. 1) in special optical schemes for controlling the parameters of counter directional beams; 2) in ring lasers to excite a unidirectional oscillation in a predetermined direction; 3) in laser gyroscopes to exclude frequency capture of the counter directional waves; and 4) in fiber optical gyroscopes to initialize the phase shift between the counter directional waves. See the following references, the teachings of which are hereby incorporated herein by reference:    1. A. A. Betin, S. C. Matthews, and M. S. Mangir, “Phase Conjugation of Depolarized Light with a Loop PC”, Nonlinear Optics: Materials, Fundamentals, and Applications Conference, Kauai, Hi., July 1998.    2. A. A. Betin and M. S. Mangir, “Loop Phase-Conjugate Mirror for Depolarized Beams,” U.S. Pat. No. 5,729,380; assigned to Hughes Electronics, March 1998.    3. S. M. Jarrett, M. W. Leitner. “Unidirectional ring laser apparatus and method,” U.S. Pat. No. 4,194,168; assigned to Spectra Physics, March 1980.    4. M. Kestigian, R. Mcclure, E. Vaher, “Magnetic mirror for imparting non-reciprocal phase shift,” U.S. Pat. No. 4,195,908; assigned to Sperry Corp., April 1980.    5. R. D. Henry, “Ring laser having magnetic isolation of counter-propagating light waves,” U.S. Pat. No. 4,219,275; assigned to Rockwell International Corp., August 1980.    6. Copending U.S. patent application Ser. No. 10/860,468, filed Jun. 2, 2004 by A. A. Betin and O. M. Efimov entitled “Wavelength Self-Adjusted Outcoupler and Method”.    7. Copending U.S. patent application Ser. No. 10/761,720, filed Jan. 20, 2004 by A. A. Betin and O. M. Efimov entitled “Wide-angle polarization-independent narrow-band spectral filter and method”.    8. G. E. Lano and C. Pinyan “Optical isolators direct light the right way,” Laser Focus World, July 1995, p. 125.    9. J. Poirson et al “Internal reflections of the Gaussian beams in Faraday isolators,” Applied Optics, June 1997, Vol. 36, No. 18, p. 4123.    10. A. A. Betin. “Polarization insensitive Faraday attenuator,” U.S. Pat. No. 6,278,547;    11. “Optical fiber amplifiers: materials, devices, and applications,” S. Sudo, editor, Artech House Inc., 1999.
NOEs like nonreciprocal mirrors introducing differential phase shifts between counter propagating beams are usually used in laser gyroscopes to exclude frequency capture of the counter directional waves. The basic disadvantages of these elements are the restricted available phase shift and the inability of independent/separate adjustment of the wavelength and intensity of each beam.
Another NOE—the Faraday isolator—is usually used in ring lasers and in loop phase conjugate mirrors (PCM). However, the Faraday isolator used for unidirectional oscillation of laser may be imperfect, lossy and change the intensity of the beam transmitted in one of the opposite directions just enough to suppress the laser oscillation in this direction. The further improvements of the Faraday isolators result in their wide application including utilization in the loop PCMs.
As a rule, in conventional NOEs a change of the propagation conditions for one beam results in a change of the conditions for the counter-propagating beam. However, independent/separate control of beams' parameters is very desirable in many cases. Hence, a need remains in the art for a system and method for providing independent control of beams propagating in opposite directions in nonreciprocal optical elements.